There's nothing going on here other than a bit of corporate disinformation.
The promotion is not by Mamiya Japan, it's by Mamiya America Corporation, a totally separate company notwithstanding its name. MAC, as it's called, is the US distributor for mamiya cameras AND for Leaf backs.
They have put together a promotional package of the two products.
That's the full story. The announcement though makes it appear that the package is from Mamiya in Japan, the camera's manufactuer, (who as some may know is in a joint venture to make cameras on an OEM basis for Phase One).
Michael